Technical fields of application of the embodiments described here are in particular radiation-sensitive cameras as they are used in transmission systems or x-ray or computer tomography systems. Examples for this are so-called “flat-panel converters”, wherein radiation is converted into visible light via an x-ray sensitive scintillator screen, so that this light may be detected by means of a camera which operates in the visible range. Typically, this visible light is detected via an extensive light-sensitive matrix detector or a camera arrangement having one or several cameras and is converted into an electronic signal.
Such cameras which are used generally show a “non-linear” or generally different performance which may differ from pixel to pixel. The consequence is that the increase of the measurement signal or increase of the brightness of the image is not uniform with respect to the increase of the illumination strength or the illumination intensity. Additionally, the sensitivity and the dark signal of each pixel or each camera may be different. This different performance is for example disturbing when the thickness of the object is to be concluded from the brightness in an x-ray recording. In particular, this has a negative influence when using a plurality of electronic cameras. Here, by each camera a partial image of the x-ray sensitive screen, which is to be optically imaged, is detected and later combined into a homogenous overall image. When combining the partial images, due to the different performances of individual pixels and/or the different sensitivities of the individual cameras clear brightness leaps may be detected at the borders. As larger flat panel converters generally are combined from several modules or camera modules or matrix detectors, the described method is to be observed in particular here.
In order to compensate for this performance, typically balancing recordings are generated at different illumination strengths. By this, the individual partial images (x-ray recordings) may be factory-balanced with respect to a different performance and with respect to the different sensitivity or the different dark signal of the individual pixels and/or cameras or modules, so that a homogenous overall image may be output. The stronger the different the characteristics are, the more balancing recordings may be used with different brightnesses or intensities in order to generate a similar (e.g. linear) output signal. For x-ray cameras put together from several individual cameras or generally several modules, this means that further with increasing deviations (non-linearity) of the individual cameras or modules an increasing number of balancing recordings with different radiation intensities is useful as otherwise the boundaries of the individual images are clearly obvious in the combined recordings and thus no homogenous image may result.
It is further enabled by this method for inhomogeneities of the radiation source to be balanced via the lateral dimensions of the x-ray recording. Such a balancing is only valid for a positioning of the (inhomogeneous) radiation source or for a distance between x-ray source and x-ray camera and for a fixed set of x-ray parameters, like e.g. x-ray voltage or illumination time. Vice versa this means, if the x-ray camera is positioned in a different location or in a different distance in the inhomogeneous radiation field or if other x-ray parameters are changed, the balancing of the x-ray camera and in particular all balancing recordings have to be regenerated.
In particular with parallel computer tomography systems or with robot computer tomography systems, frequently a plurality of balancing recording that may be used. For example with robot computer tomography systems in which by means of the first robot the x-ray source is positioned and by means of the second robot the x-ray camera, so that the object to be screened is located between the x-ray source and the x-ray camera or more accurately in the x-ray cone of the x-ray source and the x-ray camera. The first and second robots are thus each moved on predefined motion tracks, so that the object to be screened may be screened or x-rayed from different angles or different positions.
As such objects to be screened or x-rayed typically comprise varying exterior dimensions, the motion tracks of the two robots are typically not parallel. Consequently, also the distance between the radiation source and the x-ray camera varies so that the camera arrangement has to be balanced again for each x-ray recording at each x-ray position. Typically, this balancing is executed before the actual recording by the two robots arms driving along the motion tracks beforehand and detecting for example ten balancing recordings for each position. A rewritten computer tomography system drives to a plurality of for example 100 or 500 positions in one recording. Based on this, for a computer tomography recording 1000 or 5000 balancing recordings ought to be determined. In the everyday operation of such a (radiation sensitive) camera or such a computer tomography system this leads to a substantial effort. Thus, there is a need for an improved approach.